Playing hide-and-seek with Oliver is one of the stranger encounters that Baldur's Gate 3's Shadow-Cursed Lands have to offer—perhaps even stranger than the giant zombie bartender who challenges you to a drinking contest. If you encounter Oliver when first entering the area, you'd probably just assume he's a weird shadow child who enjoys being sneaky, but he's actually a very important part of Halsin's quest.
At the Last Light Inn, you find a comatose Flaming Fist called Art Cullagh babbling about Thaniel, and if you report this to Halsin, he'll set you on the path to dispelling the shadow curse. To do that, you need to find Oliver. While playing hide-and-seek with him isn't necessary to the quest, it does get you a nice magical ring, so here's Oliver's location and how to beat him.
Baldur's Gate 3 Oliver location
You can find Oliver—or Thaniel's Shadow as he's also known in Halsin's quest—close to the Shadowed Battlefield waypoint in the centre of the Shadow-Cursed Lands region. From that waypoint, head east up the tree branch to find a ruined inn with Oliver hiding inside of it. Since you're going to be sneaking around, I suggest leaving your party behind at the waypoint so they don't become a nuisance, and taking your stealthiest character solo.
How to win hide-and-seek
Upon meeting Oliver, he'll invite you to play hide-and-seek, and if you agree you'll have to win twice before he offers up a reward. The trick to beating Oliver is to use the level 2 See Invisibility spell that'll reveal him to you. Another easy way to win is to get the Ersatz Eye by letting Volo try to operate on your parasite, since this gives you See Invisibility as a fixed condition. The first game is easy to win since he just runs outside and hides behind a cart to the west.
The second game is a little tricker since he'll summon his mother and father—two shadow creatures that patrol the area and try to find you. Quickly head straight inside the inn as soon as the game starts to be safe and wait for Oliver to show up back at his original position. If you speak to him, he'll give you the Ring of Shadows, which provides the level 2 Abjuration spell, Pass Without Trace.
Now the fun's over, you can tell Oliver that he needs to reunite with Thaniel as part of the quest, which will lead to you following him through a shadow portal and fighting him before he agrees to help you out.